Photo-sensitive compositions containing a compound having a naphthoquinonediazido group or a benzoquinonediazido group have been used as positive type photoresists or positive type PS (presensitized) plates utilizing a phenomenon that when irradiated with a light having a wavelength of 300 to 500 .mu.m, the compositions undergo decomposition of quinonediazido group to produce a carboxyl group whereby the compositions become from alkali-insoluble to alkali-soluble. The compounds having a naphthoquinonediazido group or a benzoquinonediazido group can be obtained by, for example, condensing naphthoquinonediazide-sulfonyl chloride or benzoquinonediazide-sulfonyl chloride with a hydroxy group-containing low molecular weight or high molecular weight compound in the presence of a weak alkali. Examples of the low molecular weight compound include hydroquinone, resorcinol, phloroglucinol, 2,4-dihydroxybenzophenone, 2,3,4-trihydroxybenzophenone, alkyl gallates, etc., and examples of the high molecular weight compound include a phenol-formaldehyde novolak resin, a cresol-formaldehyde novolak resin, polyhydroxystyrene, etc.
The photo-sensitive compositions containing the above-described photo-sensitizers usually contain a resin component in addition to those described above. This resin component is a component necessary for providing a uniform and tough coating film to be practically usable as a photoresist or presensitized plate. In addition, the resin component is required to be developable with alkalis. As such a resin, novolak resins are optimum, and a phenol-formaldehyde novolak resin and a cresol-formaldehyde novolak resin have been widely put into practical uses.
These positive type photoresists are generally remarkably excellent in resolving power as compared with photo-curable (negative type) photoresists. Thus, because of this high resolving power, the positive type photoresists have been used as an etching-resisting film (resist film) in photographic etching for the preparation of integrated circuits such as IC or LSI (large scale integrated circuit) or photographically etching a copper-laminated base plate for forming printed circuit.
With respect to the integrated circuits, increasingly high integration degree has now required to form a fine pattern of as narrow as 1 .mu.m. In such fine pattern, film properties of the resist film and its intimate adhesiveness to a base plate are of extreme importance as well as high resolving power. That is, fine lines or patterns of about 1 .mu.m are required to be strongly adhered to the base plate without delamination and formation of pinholes during the steps of developing the resist film and etching. In these regards, conventionally used photoresist compositions, particularly positive type photoresists containing a novolak resin and as a photo-sensitizer a quinonediazide compound, are not fully satisfactory, and therefore, improvements have been demanded. Such insufficiency results from the novolak resin used in the photoresist. In general, in order to obtain a film having good film properties, i.e., the film being a soft, dense and tough thin film, and having good intimate adhesiveness to a base plate, it is required that the composition contains a compound having a molecular weight to a some extent. However, novolak resins have such a defect that the polymerization degree thereof is difficult to increase (a forced increase in the molecular weight would tend to result in gelatin, thus causing difficulty in controlling). Therefore, the resulting film is fragile and shows insufficient adhesiveness to the base plate and insufficient etching resistance, thus being liable to cause various troubles such as delamination of pattern and formation of pinholes. Further, in producing novolak resins, microgels are liable to be formed, which also can lead to formation of pinholes. As other defects of the novolak resins, they have a wide molecular weight distribution and they tend to make a mixture of many isomers. For the use as a photoresist, the molecular weight distribution is desirably narrow, and hence, such novolak resins are usually subjected to separation procedure to obtain novolak resin having a narrow molecular weight distribution. However, this procedure requires much labors and time, and even after the separation procedure, the resulting novolak resins are often still unsatisfactory. An attempt to obtain novolak resins having a narrow molecular weight distribution by controlling the production condition has been found to be not practical due to the difficulty in controlling. The wide molecular weight distribution or presence of a mixture of many isomers causes such a disadvantage that contrast (usually expressed as .gamma. value) which is one of the important properties of photoresist is low. Low contrast results in non-sharp patterns and formation of "reverse trapezoidal" or "semicircular" patterns, which leads to seriously deteriorated resolving power and etching resistance, causing formation of pinholes and reduction in yield.
A further defect of the novolak resin is that quality control of the resin is difficult.
As described hereinbefore, novolak resins, which are liable to be gelled, have a wide molecular weight distribution, and which are liable to contain many isomers, are difficult to produce products with stable quality, and hence, lot-to-lot scatter in resist performance is so wide, which is extremely disadvantageous for users.
On the other hand, the pattern line width goes on becoming finer with an increase of the integration degree of LSI. As one means for achieving it, a dry-etching technique has become popular in place of the conventional wet-etching technique.
There are many and different sorts among the dry-etching techniques. Among them, at present a so-called reactive ion-etching method is most popularly employed. This method is an anisotropic etching method combining a sputtering effect by ions having a directional property with the chemical etching by reactive plasma, and is suitable for obtaining an extremely fine pattern. A characteristic of this method is that: since not only it does not require the treatment of a waste liquor, such as hydrofluoric acid, etc., which is essential in the case of the wet-etching method but also it is free from a side-etching phenomenon which is often found in the wet-etching, a fine pattern can be readily obtained. Further, although it has hitherto been said that the dry-etching has such a defect that the speed of treating a wafer is slow as compared with the wet-etching, this defect is being overcome by improving apparatus to be used and the like. Accordingly, at present the reactive ion-etching method is an indispensable technique in increasing the integration degree.
On the other hand, from the standpoint of resists, the resist per se is being increasingly required to have dry-etching resistance with the spread of the dry-etching technique. In the dry-etching, the resist is required to be enduring, i.e., to keep a sharp pattern shape.
In general, selectivity can be employed as an index of the dry-etching resistance. This is defined to be a ratio of the etching speed of a ground (a species to be etched) to that of a resist, and it is preferred that this ratio is as high as possible.
The selectivity of presently used positive type photoresists is not always satisfactory, which, however, changes depending on the types of a ground (a species to be etched) and an etching gas. In particular, in the case that the ground is made of aluminum or a specific metal (such as molybdenum or tungsten or silicides thereof), the conditions required for the dry-etching become severe, leading to a reduction in the selectivity, and the presently available positive type photoresists are difficult to have a sufficient dry-etching resistance.
As the result of extensive investigations, the present inventors found that there is an extremely close relationship between the dry-etching resistance and heat resistance. That is, when the heat resistance of the resist is low, it is likely deformed or carbonized during the dry-etching. It may be considered that this is caused by the generation of a considerable amount of heat during the dry-etching.
Further, there is a relationship between the dry-etching resistance and heat resistance in another sense. That is, in order to improve the dry-etching resistance, a method for increasing the temperature of the so-called post-baking which is usually carried out after development but before etching the resist pattern is often employed. Although exact reasons for this are not yet clear, the etching resistance tends to be improved as the post-baking temperature increases. But in the case that the heat resistance of the resist is low, if the post-baking temperature is increased, the resist pattern causes sag or deformation at that stage, and further, these are accelerated at the dry-etching, leading to rendering it impossible to obtain desired etching results.
Accordingly, what the heat resistance is improved is no other than to improve the dry-etching resistance.